At first you have to know the dangers of being in the astral projection.
You have to have your emotions under control. When you can't you have the possibility of getting trapped in the astral projection.
You can cast a circle to protect you from evil spirits. Here is an very easy one: http://www.spellsofmagic.com/coven_ritual.html?ritual=1298&coven=6 .
For me it helped knowing my spirit animal to protect me from the dangers in the astral protection.
I hope this will help you.
The astral planes are not from a different universe, they're in this one. Traveling between planes is actually exploring this universe on different frequencies of energy which is what might cause a distortion or change of visual perception. This being said it's no more dangerous to astral project for reasons regarding outside forces than it is to not project. This is of course within the consideration of the fact that you yourself might not be currently existing in a dangerous location. Typically if a place is dangerous [on our frequency] then it is dangerous on others. For example if you traveled to Detroit via astral body or car you're still going somewhere dangerous. At the same time if you traveled to a Shaolin Temple via astral projection or by hiking, you're also less likely to get mugged.